The present invention relates to an improvement in force generating actuator control design and specifically improves control of force generation in non-linear operating regions.
Hydraulic systems have typically been the basis for generating force to such vehicle systems as braking systems, clutches, etc., especially automotive braking systems. Hydraulic systems are used to convert fluid pressure into linear and/or mechanical motion. Such systems allow the source of the hydraulic pressure to be positioned remotely from the cylinders that affect the braking action. These systems comprise an actuator, such as a brake pedal, a reservoir fluid that is responsive to pressure applied by the actuator, (such as a master cylinder) and means for converting the hydraulic pressure to a braking force, generally fluid cylinders. Mechanical braking pressure is achieved by utilizing the force of the depression of the brake pedal by the driver to increase the pressure on the master cylinder. Such systems are typically accompanied by a vacuum boost that multiplies the force supplied to the brake pedal, throughout the braking operation. The increased pressure in the master cylinder is then transmitted through fluid lines to the fluid cylinders. The fluid cylinders operate the calipers thereby forcing the calipers and brake pads against the rotors and/or drums which slows the vehicle by frictional force.
Hydraulic systems of the above-described type have many disadvantages. These include the large amount of volume and mass that the master cylinder vacuum booster, ABS modulator and hydraulic line add to the completed vehicle. Installation of standard hydraulic braking systems is also complicated and labor intensive. Additionally, the large number of parts and installation also adds to repair and maintenance issues as individual parts reach the end of their useful life. Standard hydraulic braking systems have also become dependent on the vacuum boost to assist in braking operations. However, vehicles such as electric or hybrid vehicles do not produce vacuum as a by-product of the vehicle operation. Thus vacuum boost is not an option on such vehicles.
Electric brake systems have been developed in order to overcome some of the hydraulic system disadvantages. While there are many variant forms, including electrical hydraulic systems, such an electrically operated brake system is also referred to as a brake-by-wire brake system (BBW). BBW describes the ability to activate vehicle wheel brakes via an electric signal generated by an onboard processor/controller as a result of input signals thereto. Brake torque is applied to the wheels without direct mechanical interaction between the vehicle""s brake pedal and the wheel brake.
One particular type of BBW system operates when a driver inputs a force to the brake pedal. A force sensor and/or travel sensor attached to the pedal transmits an electronic signal to an electronic controller, which in turn sends the signal to the self contained braking device typically located at each wheel of the vehicle. One such system is a hybrid system wherein electric signals are used to generate the type and amount of braking force required at each wheel of the vehicle with electrical wires rather than standard hydraulic brake lines. Located at each corner of the vehicle is a self-contained module that receives the electrical signal and mechanically brakes the vehicle. The self-contained module utilizes an individual motor that drives a ball screw piston assembly that, in turn, pressurizes hydraulic brake fluid to ultimately apply the brake caliper to a rotor at that corner of the vehicle. Another type system employs self-contained electric caliper modules that utilize an individual motor to directly apply the brake caliper to the rotor without the use of hydraulics. These types of modular BBW systems significantly reduce assembly cost. The individual modules can be separately assembled prior to the manufacture of the vehicle. The modules then only need to be bolted to the automobile during the assembly process and plugged in using standard electrical connections. Finally, the elimination of hydraulic lines stretching throughout the vehicle as well as the elimination of the master cylinder booster, and ABS modulator reduces space requirements within the engine compartment.
Due to the modularity of the BBW system, each of the individual components is preferably kept relatively small while still meeting a baseline brake response. Such a system keeps the BBW module a manageable size and does not overextend the existing electrical system on a vehicle. A modular BBW system thus works well in most brake system applications. All brake systems have linear and non-linear ranges of operation. Previous brake systems employing a central master cylinder have typically been designed such that the expected range of operation falls within its linear operating range. Since the desire is to minimize the physical size of BBW systems, these systems must operate over a wider area of the available range. Thus, in addition to the linear range of the system, these systems must also operate within the system""s non-linear range.
As in a standard hydraulic brake system, an operator pressing an input device such as a brake pedal generates the initial input to the braking system. However, instead of creating a system-wide hydraulic pressure signal to the individual brakes, pressing the brake pedal in a BBW system generates a corresponding electric input signal to a controller. This electrical input signal, in turn, is applied to a control law and a corresponding electric output signal is sent from the controller to the individual brakes. Since the BBW brake system operates over an entire non-linear range, previous methods of control law utilization are not practical. If only a linear control law is utilized, then operation in the non-linear range will cause brake system to either overshoot or result in a decreased response. Conversely, if a non-linear control law is designed for application by the controller, throughput requirements and sophistication of the controller are significantly increased. Neither option furthers the objective to have a brake system that is simple to control over both linear and nonlinear ranges of operation and maintains controller complexity at a minimum.
The present invention is aimed at one or more of the problems identified above.
In one aspect, the present invention includes a force generating apparatus for providing a force to a moving element based upon receipt of an electrical force signal. The force generating apparatus includes a force applying element coupled to the moving element for applying the force to the moving element and an actuator coupled to the force applying element for actuation thereof in response to receiving the electrical force signal. The force generating apparatus further includes a controller for determining when the force applying element is in an apply mode, determining the elapsed time until reversion to a normal mode, determining when the elapsed time is greater than a predefined minimum, modifying the value of the electrical force signal sent to the actuator, and modifying a stored control parameter.
Another aspect of the present invention is a method for modifying the electrical force signal to an actuator of a force generating apparatus wherein the force generating apparatus having a force applying element coupled to the moving element for applying the force and an actuator coupled to the force applying element for actuation thereof in response to receiving an electrical force signal from a controller. The method includes the steps of receiving a desired force actuation signal at the controller and then determining when the force generating apparatus is in an apply mode. Upon determining when the force generating apparatus is in the apply mode the length of time in the apply mode is then calculated. The calculated time value is compared to a predefined minimum associated with a normal mode and then modifying the electrical force signal according to a first predefined function when the length of time is greater than the predefined minimum. Finally, the modified electrical force signal is sent to the actuator.
Yet another aspect of the present invention is a method for modifying the electrical force signal to an actuator of a force generating apparatus. The force generating apparatus has a force applying element coupled to a moving element for applying the force and an actuator coupled to the force applying element for actuation thereof in response to receiving an electrical force signal from a controller. The method includes the steps of receiving a desired force actuation signal at the controller and then determining when the force generating apparatus is in an apply mode. The length of time the force generating apparatus is in the apply mode is calculated to obtain a time value which is then compared to a predefined minimum associated with a subsequently occurring mode other than an apply mode. The electrical force signal is modified according to a first predefined function when the calculated time value is greater than the predefined minimum. The modified electrical force signal is then sent to the actuator. The calculated time value is decremented by a predefined first constant to obtain a new time value. The new time value is compared to the predefined minimum. When the new time value is greater than the minimum time value incremented by a predefined second constant, a control law command value is modified by a second predefined function wherein the electrical force signal is at least partially a function of the control law command value. The method is reiterated in an automatic manner after every passage of a predefined time interval.
These and other features and advantages of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.